1. Field of the Invention
The present invention relates to a ferroelectric thin film and a method for producing the same. In particular, the present invention relates to a ferroelectric thin film comprising a perovskite compound (BaTiO.sub.3) and a method for producing the same. The present invention can be applied to a ferroelectric thin film element such as a memory element which utilizes reversal of polarization, a light switch which utilizes the electro-optical effect, a light modulation element, a light receiving element which utilizes the pyroelectric effect or a thin layer capacitor which utilizes a large dielectric constant.
2. Description of the Related Art
Hitherto, a BaTiO.sub.3 thin layer has been prepared as a polycrystal thin layer by a vacuum deposition method comprising evaporating raw material oxides and depositing the thin layer, or a sputtering method (cf. Japanese Journal of Applied Physics, 24 (1985), supplement 24-2,401-403 and Ferroelectrics, 37 (1981) 681-684).
Since BaTiO.sub.3 exhibits ferroelectricity at a temperature of 120.degree. C. or lower and has good ferroelectric and electro-optical characteristics, many proposals have been made on the application of BaTiO.sub.3 to various devices. However, conventionally produced BaTiO.sub.3 has the following defects:
The crystal structure of BaTiO.sub.3 is a tetragonal system. Since its polarization axis is on the c axis, it has large anisotropy of properties between the c axis and other directions which are perpendicular to the c axis. Then, in some crystal orientations, various properties such as the reversal of polarization, the electro-optical effect and the pyroelectric effect cannot be used, and therefore, it is desired to use a material having a specific orientation depending on the kind of the device.
Since BaTiO.sub.3 thin film prepared by the conventional production method is a polycrystal and its properties are averaged because of polycrystallinity, the thin film as a whole does not have good properties. Further, the polycrystal BaTiO.sub.3 thin film requires high operation voltage.
Since the conventional BaTiO.sub.3 is a polycrystal, it has grain boundaries and poor surface smoothness, the light is scattered at the grain boundary and the surface when it is used as an optical element. This leads to increase of propagation loss of the light.
The conventional vacuum evaporation method uses a so-called flash evaporation method in which a once sintered BaTiO.sub.3 ceramics is ground to a particle size of several millimeters or less and several particles are dropped on a tungsten heater kept at 2000.degree. C. or higher to evaporate the material.
Therefore, in such method, it is difficult to control the composition of the thin film, and contamination of the thin film with tungsten from the heater cannot be avoided. In addition, since it is difficult to control the conditions for growing the crystal, reproducibility and productivity of such method are not good.
In the sputtering method, it is not easy to produce a multi-component target such as BaTiO.sub.3. Because of difference of sputtering rates among the elements, it is difficult to prepare a thin film having a homogeneous composition. Since evaporation rates are different among the elements due to the difference of sputtering rates and increase of temperature on the target surface caused by bombardment with argon ions, the composition on the target surface greatly changes as time passes. Further, since the formed thin film is exposed to the argon plasma, the crystallinity of the thin film is disturbed.
In addition, none of the BaTiO.sub.3 thin films prepared by the above methods exhibits ferroelectricity in general. To prepare a ferroelectric BaTiO.sub.3 thin film, a substrate should be heated to a temperature exceeding 1000.degree. C. during formation of the thin film, or the formed thin film should be annealed at a temperature exceeding 1000.degree. C. However, with such thin films, contamination with impurities or great decrease of electric resistance because of oxygen defects have been reported.